Double-Armed 18- and 21-Membered Macrocycles as Potential Chelators for Lead and Bismuth Radiopharmaceuticals

With increasing clinical applications and interest in targeted alpha therapy, there is growing interest in developing alternative chelating agents for [²¹²Pb]Pb²⁺ and [^212/213Bi]Bi³⁺ that exhibit rapid radiolabeling kinetics and kinetic inertness. Herein we report the synthesis and detailed investigation of diacetate and dipicolinate 18- and 21-membered macrocyclic chelators BADA-18, BADA-21, BADPA-18, and BADPA-21 for the complexation of Pb²⁺ and Bi³⁺ ions with potential use in the preparation of radiopharmaceuticals. The formation of mononuclear complexes was established by using ESI-mass spectrometry, and their stability constants were determined by potentiometric titration. A thorough study of the structure of the metal complexes was carried out by using X-ray diffraction and NMR spectroscopy. It was shown how the stability of the complex is influenced by an increase in the size of the macrocycle, the replacement of acetate arms with picolinate ones, the rigidity of the ligand, as well as the type of conformation (syn- or anti-) of the metal complex. The new ligands were radiolabeled with [²¹⁰Pb]Pb²⁺ and [²⁰⁷Bi]Bi³⁺, and the in vitro stability of the resulting complexes in a competitive environment of serum and biologically significant metal ions was assessed. Rapid complex formation in 1-2 min at room temperature, as well as the high kinetic inertness of the complexes Pb(BADPA-18) and Bi(BADPA-18) in biological media, demonstrate its potential for use in targeted radionuclide therapy.